Why would a solid, promising pharmaceutical play like Pfizer (PFE) ever be interested in acquiring a problem-riddled, shaky company like AstraZeneca (AZN)?

First and foremost, know that Pfizer’s rumored acquisition interest in AstraZeneca appears to have come and gone without anyone noticing that the two companies met several months ago to discuss a union.

While nothing is ever final, those meetings ended with no further actions planned, and no plans for future discussions of such a deal that might boost the value of Pfizer stock. The odds of an acquisition panning out after a failed effort are even weaker than they are the first time the idea came up.

That being said, where there’s smoke, there’s fire. If Pfizer was interested in a then-struggling AstraZeneca a few months ago, with no major changes to the company’s structure, properties, and pipeline in the meantime, there’s no reason to think the iconic pharmaceutical maker would be unwilling to entertain the idea again now.

The million-dollar question Pfizer stock holders as well as AZN stock owners are asking themselves now is, what exactly did Pfizer see in AstraZeneca in the first place? As it turns out, the three best explanations are pretty straightforward. But are they enough reason to go through with the deal?

The motivations are (in order of most-likely to less-likely):

AstraZeneca Has a Solid Cancer-Immunology Pipeline

While Pfizer has done a reasonably good job of refilling its pipeline and product portfolio following the patent expirations of Lipitor and Viagra just to name a few, one of the biopharma areas where it’s lacking is on the cancer front, and on the immunology front in particular. That has made some investors worried about the future of Pfizer stock.

AstraZeneca has a compelling, though not quite outstanding, development pipeline of cancer-immunology drugs. Granted, the biggest reason AstraZeneca has such a robust oncology-immunology pipeline is because it acquired Amplimmune in 2013, but it’s still under the AstraZeneca umbrella.

The shining star in the early-stage pipeline is tremelimumab, for mesothelioma. It’s only in Phase 2 trials right now, however, spurring concerns that the company doesn’t have much that could be brought to market in the foreseeable future. But those are errant assumptions. Olaparib (for ovarian and breast cancer), moxetumomab (for leukemia), and selumetinib (for non-small cell lung cancer) are all Phase 3 candidates that could provide some real support for the value of Pfizer stock.